Surface chemistry, structure, and reactivity
of multi-component spinel nanoparticles
Subproject P10
Multi-component spinel oxides are complex materials. Understanding their properties and reactivity is challenging, even more so when considering defect-rich nanoparticles under actual reaction conditions.
In P10, we will apply a comprehensive, multi-technique operando approach to investigate Fe-based spinel oxide nanoparticles used as WGS and oxidation catalysts in the gas and liquid phase. We will determine their surface composition, particularly under reaction conditions, the state, coordination environment, and role of the constituent cations, and the influence of defects. We will link these properties to the reactivity and interaction with O2, H2O, H2, CO, and CO2. Furthermore, we will evaluate how spinels and their surfaces change when exposed to the liquid phase. Our experimental approach comprises synthesis, characterization (TEM, XRD, XPS, TPD, titration of sites and defects, IR of probe molecules), steady-state and transient kinetics, and operando characterization (IR, NAP-XPS, XAS).
In close interaction with surface science (P04 Parkinson), we will compare the nanoparticulate materials to single-crystal and thin-film model systems. For understanding complex materials, a close collaboration with the surface science and theory groups is essential. In return, our results on technologically relevant nanoparticles under operation conditions will help to validate and adapt models and address the influence of high defectivity, low coordinated sites, disorder, and low crystallinity. We aim to bridge fundamental theory studies, surface science experiments, and model studies (P11 Backus) towards real-world application.
Expertise
Our group has long term experience in the application of operando spectroscopy (FTIR, XPS and XAS) for studying heterogeneous catalysts. Our research interests are centered around establishing structure-performance relations of oxides and supported metal nanoparticles and identifying reaction mechanisms. Understanding the elementary reaction steps occurring at the catalyst surface and identification of the involved intermediates and surface sites under relevant conditions is a main focus and crucial for a rational design and improvement of catalytic materials.
Methods and expertise available in our lab include:
- in situ/operando FTIR (transmission, DRIFTS and ATR-IR) during catalytic reactions (steady-state and concentration modulation setups)
- several laboratory-scale flow reactors equipped with gas chromatographs and mass spectrometers for performing catalytic reactions in the gas and liquid phase
- in-situ Near Ambient Pressure XPS setup
- volumetric physisorption and chemisorption, dynamic (pulsed) chemisorption
- temperature-programmed methods (TPD, TPR, TPO)
- DR-UV/VIS spectroscopy
- thermal analysis (DSC and TGA)
- fully equipped synthesis lab
- we regularly perform in situ XAS and high resolution XRD/total scattering at synchrotron facilities using dedicated operando cells
- we frequently utilize HR-TEM with EDX and EELS, SEM, XRF, XRD (including in situ XRD) and ICP-MS available via service centers and/or collaborations
Team
Associates
Publications
2016
Lukashuk, Liliana; Föttinger, Karin; Kolar, Elisabeth; Rameshan, Christoph; Teschner, Detre; Hävecker, Michael; Knop-Gericke, Axel; Yigit, Nevzat; Li, Hao; McDermott, Eamon; Stöger-Pollach, Michael; Rupprechter, Günther
Operando XAS and NAP-XPS studies of preferential CO oxidation on Co3O4 and CeO2-Co3O4 catalysts
Journal ArticleOpen AccessIn: Journal of Catalysis, vol. 344, pp. 1–15, 2016.
Abstract | Links | BibTeX | Tags: P08, P10, pre-TACO
@article{Lukashuk2016,
title = {Operando XAS and NAP-XPS studies of preferential CO oxidation on Co_{3}O_{4} and CeO_{2}-Co_{3}O_{4 }catalysts},
author = {Liliana Lukashuk and Karin Föttinger and Elisabeth Kolar and Christoph Rameshan and Detre Teschner and Michael Hävecker and Axel Knop-Gericke and Nevzat Yigit and Hao Li and Eamon McDermott and Michael Stöger-Pollach and Günther Rupprechter},
doi = {10.1016/j.jcat.2016.09.002},
year = {2016},
date = {2016-12-01},
urldate = {2016-12-01},
journal = {Journal of Catalysis},
volume = {344},
pages = {1--15},
publisher = {Elsevier BV},
abstract = {Co_{3}O_{4} is a promising catalyst for removing CO from H_{2} streams via the preferential CO oxidation (PROX). A Mars-van-Krevelen redox mechanism is often suggested but a detailed knowledge especially of the oxidation state of the catalytically active surface under reaction conditions is typically missing. We have thus utilized operando X-ray absorption spectroscopy to examine structure and oxidation state during PROX, and near atmospheric pressure-XPS at low photoelectron kinetic energies and thus high surface sensitivity to monitor surface composition changes. The rather easy surface reduction in pure CO (starting already at ∼100 °C) and the easy reoxidation by O_{2} suggest that molecularly adsorbed CO reacts with lattice oxygen, which is replenished by gas phase O_{2}. Nevertheless, the steady state concentration of oxygen vacancies under reaction conditions is too low even for XPS detection so that both the bulk and surface of Co_{3}O_{4} appear fully oxidized during PROX. Furthermore, the effect of adding CeO_{2} (a less active material) to Co_{3}O_{4} was studied. Promotion of Co_{3}O_{4} with 10 wt% CeO_{2} increases the reduction temperatures in CO and H_{2} and enhances the PROX activity. Since CeO_{2} is a less active material, this can only be explained by a higher activity of the Co-O-Ce interface.},
keywords = {P08, P10, pre-TACO},
pubstate = {published},
tppubtype = {article}
}
Wolfbeisser, Astrid; Sophiphun, Onsulang; Bernardi, Johannes; Wittayakun, Jatuporn; Föttinger, Karin; Rupprechter, Günther
Methane dry reforming over ceria-zirconia supported Ni catalysts
Journal ArticleOpen AccessIn: Catalysis Today, vol. 277, pp. 234–245, 2016.
Abstract | Links | BibTeX | Tags: P08, P10, pre-TACO
@article{Wolfbeisser2016,
title = {Methane dry reforming over ceria-zirconia supported Ni catalysts},
author = {Astrid Wolfbeisser and Onsulang Sophiphun and Johannes Bernardi and Jatuporn Wittayakun and Karin Föttinger and Günther Rupprechter},
doi = {10.1016/j.cattod.2016.04.025},
year = {2016},
date = {2016-11-15},
urldate = {2016-11-15},
journal = {Catalysis Today},
volume = {277},
pages = {234--245},
publisher = {Elsevier BV},
abstract = {Nickel nanoparticles supported on Ce_{1-x}Zr_{x}O_{2} mixed oxides prepared by different synthesis methods, as well as Ni-ZrO_{2} and Ni-CeO_{2}, were evaluated for their catalytic performance in methane dry reforming (MDR). MDR is an interesting model reaction to evaluate the reactivity and surface chemistry of mixed oxides. Textural and structural properties were studied by N_{2} adsorption and XRD. Mixed oxide preparation by co-precipitation resulted in catalysts with higher surface area than that of pure ZrO_{2} or CeO_{2}. XRD analysis showed the formation of different Ce_{1-x}Zr_{x}O_{2} solid solutions depending on using a surfactant or not. The catalyst prepared by surfactant assisted co-precipitation was not active for methane dry reforming most likely because of the encapsulation of Ni particles by ceria-zirconia particles, as revealed by TEM and H_{2} chemisorption. The catalytic activity of the catalyst prepared by co-precipitation without surfactant was comparable to Ni-ZrO_{2}. Clearly, catalyst activity strongly depends on preparation and on the resulting phase composition rather than on nominal composition. Compared to Ni-ZrO_{2} the ceria-zirconia supported Ni catalyst did not achieve higher activity or stability for methane dry reforming but, nevertheless, the formation of filamentous carbon was strongly reduced (100 times less carbonaceous species). Consequently, using ceria-zirconia as a support material decreases the risk of reactor tube blocking.},
keywords = {P08, P10, pre-TACO},
pubstate = {published},
tppubtype = {article}
}
2014
Föttinger, Karin; Rupprechter, Günther
Journal ArticleIn: Accounts of Chemical Research, vol. 47, no. 10, pp. 3071–3079, 2014.
Abstract | Links | BibTeX | Tags: P08, P10, pre-TACO
@article{Foettinger2014,
title = {In Situ Spectroscopy of Complex Surface Reactions on Supported Pd–Zn, Pd–Ga, and Pd(Pt)–Cu Nanoparticles},
author = {Karin Föttinger and Günther Rupprechter},
doi = {10.1021/ar500220v},
year = {2014},
date = {2014-09-23},
journal = {Accounts of Chemical Research},
volume = {47},
number = {10},
pages = {3071--3079},
publisher = {American Chemical Society (ACS)},
abstract = {It is well accepted that catalytically active surfaces frequently adapt to the reaction environment (gas composition, temperature) and that relevant “active phases” may only be created and observed during the ongoing reaction. Clearly, this requires the application of in situ spectroscopy to monitor catalysts at work. While changes in structure and composition may already occur for monometallic single crystal surfaces, such changes are typically more severe for oxide supported nanoparticles, in particular when they are composed of two metals. The metals may form ordered intermetallic compounds (e.g. PdZn on ZnO, Pd_{2}Ga on Ga_{2}O_{3}) or disordered substitutional alloys (e.g. PdCu, PtCu on hydrotalcite). We discuss the formation and stability of bimetallic nanoparticles, focusing on the effect of atomic and electronic structure on catalytic selectivity for methanol steam reforming (MSR) and hydrodechlorination of trichloroethylene. Emphasis is placed on the in situ characterization of functioning catalysts, mainly by (polarization modulated) infrared spectroscopy, ambient pressure X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and X-ray diffraction. In the present contribution, we pursue a two-fold, fundamental and applied, approach investigating technologically applied catalysts as well as model catalysts, which provides comprehensive and complementary information of the relevant surface processes at the atomic or molecular level. Comparison to results of theoretical simulations yields further insight.
Several key aspects were identified that control the nanoparticle functionality: (i) alloying (IMC formation) leads to site isolation of specific (e.g. Pd) atoms but also yields very specific electronic structure due to the (e.g. Zn or Ga or Cu) neighboring atoms; (i) for intermetallic PdZn, the thickness of the surface alloy, and its resulting valence band structure and corrugation, turned out to be critical for MSR selectivity; (ii) the limited stability of phases, such as Pd_{2}Ga under MSR conditions, also limits selectivity; (iii) favorably bimetallic catalysts act bifunctional, such as activating methanol AND water or decomposing trichlorothylene AND activating hydrogen; (iv) bifunctionality is achieved either by the two metals or by one metal and the metal–oxide interface; (v) intimate contact between the two interacting sites is required (that cannot be realized by two monometallic nanoparticles being just located close by).
The current studies illustrate how rather simple bimetallic nanoparticles may exhibit intriguing diversity and flexibility, exceeding by far the properties of the individual metals. It is also demonstrated how complex reactions can be elucidated with the help of in situ spectroscopy, in particular when complementary methods with varying surface sensitivity are applied.},
keywords = {P08, P10, pre-TACO},
pubstate = {published},
tppubtype = {article}
}
Several key aspects were identified that control the nanoparticle functionality: (i) alloying (IMC formation) leads to site isolation of specific (e.g. Pd) atoms but also yields very specific electronic structure due to the (e.g. Zn or Ga or Cu) neighboring atoms; (i) for intermetallic PdZn, the thickness of the surface alloy, and its resulting valence band structure and corrugation, turned out to be critical for MSR selectivity; (ii) the limited stability of phases, such as Pd2Ga under MSR conditions, also limits selectivity; (iii) favorably bimetallic catalysts act bifunctional, such as activating methanol AND water or decomposing trichlorothylene AND activating hydrogen; (iv) bifunctionality is achieved either by the two metals or by one metal and the metal–oxide interface; (v) intimate contact between the two interacting sites is required (that cannot be realized by two monometallic nanoparticles being just located close by).
The current studies illustrate how rather simple bimetallic nanoparticles may exhibit intriguing diversity and flexibility, exceeding by far the properties of the individual metals. It is also demonstrated how complex reactions can be elucidated with the help of in situ spectroscopy, in particular when complementary methods with varying surface sensitivity are applied.